1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular drive system including an electrically controlled differential portion having a differential mechanism capable of performing a differential function, and a transmission portion disposed in a power transmitting path between the electrically controlled differential portion and a drive wheel of a vehicle, and more particularly to techniques for reducing a load of the vehicular drive system in a high-load operating condition.
2. Discussion of Prior Art
There is known a control apparatus for a vehicular drive system including (a) an electrically controlled differential portion having a differential mechanism and operable to control a differential state between its input and output speeds by controlling an operating state of an electric motor connected to a rotary element of the differential mechanism, and (b) a transmission portion which constitutes a part of a power transmitting path between the electrically controlled differential portion and a drive wheel of a vehicle and which functions as an automatic transmission. mechanism, and an electric motor which is connected to a rotary element of
JP-2005-237491A discloses an example of a control apparatus for a vehicular drive system including (a) an engine, (b) and an electrically controlled differential portion having a first electric motor, a second electric motor, a power transmitting member, and a planetary gear set (differential mechanism) which includes a first rotary element connected to the engine, a second rotary element connected to the first electric motor and a third rotary element connected to the second electric motor, and which is arranged to distribute an output of the engine to the first electric motor and the power transmitting member, and (c) a transmission portion which constitutes a part of a power transmitting path between the power transmitting member and a drive wheel of a vehicle and which functions as a step-variable automatic transmission.
The control apparatus for the vehicular drive system disclosed in the above-identified publication is arranged to change or control a speed ratio of the electrically controlled differential portion, so as to reduce a stepping change of an overall speed ratio of the vehicular drive system due to a stepping change of a speed ratio of the transmission portion caused by a shifting action thereof, so that the overall speed ratio defined by the speed ratio of the electrically controlled differential portion and the speed ratio of the transmission portion is continuously changed, and so that a stepping change of the operating speed of the engine during the shifting action of the transmission portion. Thus, the vehicular drive system as a whole can function as a continuously variable transmission which performs a so-called “iso-power shifting” in which the operating state of the engine (represented by its speed and torque, for example) is held substantially constant during the shifting action of the transmission portion, so that the vehicle drive power sources such as the engine and the electric motor are operated with high efficiency.
In the vehicular drive system described above, the transmission portion is usually shifted according to a predetermined shifting map. In some running condition of the vehicle, however, a special shifting action of the transmission portion is required to reduce a load of the vehicular drive system, that is, to improve the operating efficiency of the vehicle drive power sources. For instance, the load of the vehicular drive system becomes high when the electric motor is operated in an operating condition of low efficiency, or when the transmission portion is placed in a low-efficiency gear position in which a comparatively large amount heat is generated. An increase of the load of the vehicular drive system tends to cause a temperature rise of the electric motor, and a rise of the temperature of a working fluid used to operate the transmission portion and lubricate and cool the various parts of the vehicular drive system such as the electric motor. In view of this drawback, the special shifting action of the transmission portion is required for the purpose of changing the operating condition of the electric motor for improved operating efficiency and reduced operating load, and reducing the load of the transmission portion to thereby reduce the amount of heat generation, in order to reduce the load of the vehicular drive system, so that the temperature rises of the electric motor and the working fluid.
Unlike the ordinary shifting action of the transmission portion performed according to the predetermined shifting map, the special shifting action of the transmission portion for the purpose of reducing the load of the vehicular drive system has a risk of an excessive rise of the rotating speeds of rotary elements of the vehicular drive system beyond a limit value, and consequent deterioration of the durability of the vehicular drive system. For example, a certain relationship between the input speed of the transmission portion and the operating speed of the engine after the special shifting action of the transmission portion may cause a rise of the operating speed of the electric motor determined by relative operating speeds of the rotary elements of the differential portion, resulting in deterioration of the durability of the electric motor, or may cause a rise of the rotating speed of a pinion gear of the differential mechanism (in other words, an increase of a difference between the engine speed and the input speed of the transmission portion), resulting in deterioration of the durability of the pinion gear (durability of its needle bearing and bushing).
Although it is desirable to perform the special shifting action of the transmission portion for reducing the temperature rises of the electric motor and the working fluid, it is required to limit the special shifting action that causes a rise of the rotating speeds of the rotary elements beyond the upper value. However, the limitation of the special shifting action leads to a failure to lower the temperatures of the electric motor and the working fluid, also giving rise to a risk of deterioration of the durability of the vehicular drive system.